3GPP Long Term Evolution (LTE) Multiple Input Multiple Output (MIMO) system and related codebook enabled spatial multiplexing are designed basically based on co-located transmit antenna port configurations, or on the assumption that there is no average power mismatch between the propagation paths. For such a scenario where receive antenna ports are well-balanced in terms of the received power, considerable MIMO gain can be expected related to the usage of corresponding precoders and consequently a satisfactory throughput.
However, in the case of geographically separated transmit antenna ports deployment, a receive antenna port close to one transmit antenna port and far from another one, undergoes a very large difference of the received signal powers from different transmit antenna ports.
It has been shown that received power imbalance of the signals transmitted over a MIMO channel results in reduced rank, i.e. the number of orthogonal propagation paths of the MIMO channel observed at the receiver. Thus rank 1 (or generally a low rank) reporting and transmission will be more likely in that case, which will reduce the throughput compared to the co-located antenna scheme even if the received signal power close to one antenna is considerably high.
In a real-life deployment there are several types of imperfections which can results in mismatches associated with antenna arrays creating non-balanced propagation conditions, such as mismatches due to the physical antenna system structure (i.e. spacing) or mismatches due to the hardware elements used in the transmit/receive chains for each antenna port. Another type of mismatch consists of mismatches created as an effect due to hand gripping of mobile terminals. Hence, a mismatch leads to a power imbalance seriously effecting throughput of the communication system.